With the recent development of office machines and progress of office automation, offset lithographic printing process has spread in the art of simple printing, which comprises subjecting a direct imaging lithographic printing plate precursor comprising an image-receiving layer having a hydrophilic surface provided on a water-resistant support to various processing processes, i.e., image formation to prepare a printing plate.
A conventional direct imaging lithographic printing plate comprises an image-receiving layer containing an inorganic pigment, a water-soluble resin, a waterproofing agent, etc. provided on a support such as waterproofed paper and plastic film. In accordance with a known plate-making process, a lipophilic ink is typewritten or imagewise handwritten on such a direct imaging lithographic printing plate precursor. Alternatively, an image is hot-melt transferred from an ink ribbon to such a direct imaging lithographic printing plate precursor in a heat transfer printer. Alternatively, a lipophilic image is formed on such a direct imaging lithographic printing plate precursor in an ink jet printer using a liquid ink.
Examples of the inorganic pigment to be contained in the image-receiving layer of the printing plate precursor include kaolin, clay, talc, calcium carbonate, silica, titanium oxide, zinc oxide, barium sulfate, and alumina.
Examples of the water-soluble resin include polyvinyl alcohol (PVA), modified PVA such as carboxy PVA, starch, starch derivative, cellulose derivative such as carboxymethyl cellulose and hydroxyethyl cellulose, casein, gelatin, polyvinyl pyrrolidone, vinyl acetate-crotonic acid copolymer, styrene-maleic acid copolymer or the like.
Examples of the waterproofing agent include glyoxal, melamine formaldehyde resin, precondensate of aminoplast such as urea formaldehyde resin, modified polyamide resin such as methylolated polyamide resin, polyamide--polyamine-epichlorohydrin adduct, polyamide epichlorohydrin resin, modified polyamide polyimide resin or the like.
It is known that the image-receiving layer may further comprise crosslinking catalysts such as ammonium chloride and silane coupling agent incorporated therein.
In recent plate-making by various printers, the image-receiving layer of the printing plate precursor is required to have not only hydrophilicity to prevent printing ink stain but also water resistance as the lithographic printing plate, and further sufficient adhesion to the lipophilic image layer formed thereon. Various proposals have been made to meet these requirements.
For example, a proposal has been made to improve hydrophilicity and image adhesion by the application of an image-receiving layer formed by using zinc oxide, kaolinite and alumina as the inorganic pigment, a water-soluble resin, a waterproofing agent and acetic acid in combination to prepare a dispersion, in which acetic acid reacts with zinc oxide to produce zinc acetate, and applying the dispersion to form an image-receiving layer (JP-A-63-54288) (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Another proposal has been made to use the same image-receiving layer as proposed above except that talc or silica is used instead of alumina and an aluminum-based, zirconium-based or titanium-based metal compound is used as waterproofing agent to enhance its hydrophilicity and water resistance (JP-A-63-166590 and JP-A-63-166591).
When printing plates are prepared by an electrophotographic printer using a dry toner (PPC copying machine), a toner is attached to non-image area on the printing plates thus made. The printed matters provided by these printing plates show background stain. In order to overcome the foregoing disadvantage, JP-B-6-96353 (the term "JP-B" as used herein means an "examined Japanese patent publication") proposes the use of an inorganic pigment such as silica having an average grain diameter of from 5 to 20 .mu.m to adjust the surface roughness of the image-receiving layer to a specific range. Further, JP-A-62-157058 proposes the combined use of silica and alumina sol each having an average grain diameter of from 5 to 20 .mu.m as the inorganic pigment.
As an approach for eliminating ink stain on non-image area and insufficient adhesion of ink image at the plate-making process by not only PPC copying machine but also heat-sensitive transfer printer, JP-A-6-183164 proposes the combined use of a pigment such as colloidal silica and calcium bicarbonate having a grain diameter of not more than 20 nm and a lubricant such as polyethylene wax emulsion. Further, JP-B-5-17871 proposes the combined use of a synthetic silica powder having a grain diameter of not more than 20 .mu.m, a colloidal silica having a grain diameter of not more than 50 nm and a hydrophilic polyvinyl alcohol resin.
However, the printing plates thus prepared are disadvantageous in that they have an insufficient mechanical strength of the image area and printing easily causes blanks in the image area or remarkable background stain in the non-image area.
On the other hand, the recent spread of various office automated machines, various computers and their peripheries and development of related techniques have made it possible to form an image compiled by personal computer or workstation on a lithographic printing plate directly by various printers capable of processing digital output signals.
In plate-making by an ink jet printer capable of processing digital signals, JP-A-64-27953 proposes a hot-melt ink jet process (also referred to as "solid jet process") using a liquid ink obtained by heating and melting a hydrophobic solid ink as an approach for eliminating the diffusion or absorption of an image-forming agent in a liquid ink to eliminate image stain.
However, this approach, too, is disadvantageous in that the resulting printing plates give printed matters having stain on image area and remarkable printing ink stain on background in non-image area. Further, these printing plates can print about 200 to 300 sheets at most.